ANPT L-1 Ring Gage - A Quick Overview


An ANPT L-1 Ring Gage is a special gage that is used on the external rings of devices or components. This includes plugs, pipes, or other forms of fittings that have ring threads. But contrary to other gages, the ANPT L1 ring gage is applicable on ANPT threads which are also called the Aeronautical National Pipe Taper threads.

ANPT threads are different in comparison to NPT threads as they do not require any form of sealants. Whereas NPT threads require sealants since they are not designed to be used in aeronautical applications. ANPT threads are suitable for aeronautical applications where there is a need for a sealant thread joint that is not possible with either O-rings, gaskets, or straight threads.

How To Use It?

All the external threads are first gaged with the L1 thread ring gage which travels L1 distance over the threads and engages the threads that will be engaged when the component is hand-tied. To check the threads beyond the L1 length, an L2 ring gage is used.

In essence, ANPT thread ring gages are available in three measurements:

  1. The ANPT L1 ring gage also called the thin ring is used for checking the thread elements over the length of engagement that is hand tightened
  2. The ANPT L2 ring gage also called the thick ring, measures the thread elements when the component is tightened with a wrench
  3. The final is the ANPT 6-step plain ring which is used for checking the major and minor diameter of the male.

Moreover, ANPT thread ring gages are measured on the SAE AS71051 standard.

More On ANPT L-1 Ring Gage

The process for measurement is rather simple. Simply place the ring onto the thread and rotate the gage. It should travel smoothly all the way up to the L1 distance, which are the thread elements that are engaged when the component is Hand tightened. The ANPT L1 screw threads have a larger major diameter in comparison to NPT gages. Similarly, the minor diameter is also larger, and the rings are designed such that they form a mechanical seal that eliminates the need for using any kind of sealant, making them ideal for aeronautical applications. 

ANPT threads are measured in the same fashion as NPTF threads, both internally and externally (we use the ANPT L-1 ring for external measurement). But the gages are still dimensionally different which is why they cannot be used interchangeably. From the basic number of turns specified for the ANPT L-1 ring, a turn of 1 ½ from the basic turn is still acceptable for the L2 ring gage.


At WESTport Corporation, we have been serving the industry since 1989 by providing quality assurance products and services. Our premium quality products are not only designed to be inexpensive, but unparalleled in class and quality. WESTport employs state-of-the-art measures to ensure all our products comply with the ISO 6506, ISO 6508, ISO 9001, and ISO/IEC 17025 standards. Moreover, our expanded product line includes (but is not limited to):

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  • Medical Gages
  • Spline Gages
  • Custom Gages (made to your blueprint specifications)
  • API Plug and Ring Gages
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